1. Field of the Invention
The present invention relates to an active-matrix semiconductor display device having a thin film transistor and a driving method thereof.
2. Description of the Related Art
A thin film transistor having a semiconductor film formed over an insulation surface is an essential semiconductor element for a driver circuit or a pixel portion included in an active-matrix semiconductor display device. Since manufacturing the thin film transistor is limited in terms of allowable temperature limit of a substrate, a thin film transistor including, as an active layer, amorphous silicon which can be formed at a relatively low temperature, polysilicon which can be obtained by crystallization using a laser beam or a catalytic element, or the like is mainly used as a transistor for the semiconductor display device.
In recent years, a metal oxide having semiconductor characteristics which is referred to as an oxide semiconductor has attracted attention as a novel semiconductor material which has both high mobility, which is a characteristic of polysilicon, and uniform element characteristics, which is a characteristic of amorphous silicon. The metal oxide is used for various applications. For example, indium oxide which is a well-known metal oxide is used as a material of a transparent electrode included in a liquid crystal display device or the like. The examples of such metal oxides having semiconductor characteristics are a tungsten oxide, a tin oxide, an indium oxide, a zinc oxide, and the like. A thin film transistor, a channel formation region of which is formed using such a metal oxide having semiconductor characteristics, is already known (for example, see Patent Documents 1 to 4 and Non-Patent Document 1).
As the metal oxides, not only single-component oxides but also multi-component oxides are known. For example, homologous compound, InGaO3(ZnO)m (m is natural number) is known as a multi-component oxide having In, Ga, and Zn (see Non-Patent Documents 2 to 4). Further, it is confirmed that such an oxide semiconductor made of an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (see Patent Document 5 and Non-Patent Documents 5 and 6).